En español | A small number of Americans have been infected with the coronavirus after getting the COVID-19 vaccine. Called “breakthrough cases,” they have been making headlines recently, and they raise a question: What are your chances of getting COVID-19 if you are fully vaccinated?
The answer, studies suggest, is very low — probably just a fraction of a percentage point. Still, a few breakthrough cases are inevitable, even with highly effective vaccines.
"You will see breakthrough infections in any vaccination when you're vaccinating literally tens and tens and tens of millions of people. So in some respects, that's not surprising,” Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said at a March 26 White House COVID-19 briefing.
A Centers for Disease Control and Prevention (CDC) report published on April 2 found that the two-dose COVID-19 vaccine regimen (by Pfizer-BioNTech and Moderna) prevented 90 percent of coronavirus infections two weeks after the second dose, which is when you are considered fully vaccinated. Of the 2,479 vaccinated people in the CDC study, just three had confirmed coronavirus infections after they were fully vaccinated.
Importantly, even if you do get infected after your vaccination, your case is likely to be asymptomatic or mild, like a common cold, says Gregory Poland, an infectious disease physician at the Mayo Clinic in Rochester, Minnesota, and director of Mayo's Vaccine Research Group.
Of the total 9,245 breakthrough cases reported to the CDC as of April 26, 2021, 835 resulted in hospitalization, federal data show. That’s out of more than 95 million Americans who had been vaccinated at that point.
Poland stresses that all three authorized COVID-19 vaccines — from Pfizer-BioNTech, Moderna and Johnson & Johnson — are extraordinarily effective at preventing severe illness: “I've been a vaccinologist for four decades, and I've never seen efficacy like this in first-generation vaccination.”
What efficacy rates really mean
You've probably heard about each vaccine's efficacy rate. In their clinical trials, Pfizer-BioNTech's and Moderna's two-shot vaccines had an efficacy rate of about 95 percent, while the Johnson & Johnson single-dose vaccine had a 72 percent efficacy rate in the U.S.
If a vaccine's efficacy rate is 95 percent, you might assume that 5 out of every 100 people vaccinated people will get sick. But that's not how the math works, says Anna Wald, an infectious disease physician and epidemiologist at the University of Washington School of Medicine.
The actual percentage of vaccinated people who got COVID-19 in both the Pfizer and Moderna clinical trials was far smaller — just around 0.4 percent.
Efficacy is actually calculated by comparing people in a trial who got the vaccine to people who got the placebo, Wald says. So if you received the Pfizer-BioNTech or Moderna vaccine, “whatever your chance was [of getting COVID-19] before, it's now 95 percent less,” Wald explains.
There are two more things to know about those efficacy rates. First, none of the trial participants who received any of the authorized vaccines died of COVID-19. In other words, when it comes to what's most important — preventing death — the vaccines were 100 percent effective in the trials.
Second, the trials were designed only to assess whether the vaccines could prevent someone from getting sick from COVID-19, so the researchers tested only those who developed symptoms. That means the efficacy rates from the trials don't reflect how well the vaccines worked at preventing asymptomatic infections
‘Breakthrough cases’ are extremely rare
Now that the vaccines have been authorized and are going into the arms of millions, health researchers are closely tracking their performance in the real world and also looking at whether they protect against asymptomatic infections.
"When you do a clinical trial, that is an idealized population. Everything is as perfect as it can be,” Poland says. “That is different than what we call ‘real-world effectiveness,’ where now unhealthy people are getting it, there aren't strict protocols, and you're dependent on 100,000 different vaccine administrators doing it right."
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So far, studies looking at the real-world effectiveness of the COVID-19 vaccines have found the vaccines to be highly effective at preventing both symptomatic and asymptomatic infections. A study released by the CDC in mid-May found that the Pfizer-BioNTech and Moderna vaccines reduced the risk of getting sick with COVID-19 by 94 percent among a large and diverse population of fully vaccinated health care workers.
Two studies published March 23 in The New England Journal of Medicine suggest that breakthrough cases are very rare among the fully vaccinated. One study found that just 4 of 8,121 employees at the University of Texas Southwestern Medical Center in Dallas tested positive for coronavirus after being fully vaccinated. The other found that just 7 of 14,990 California health care workers tested positive two weeks after their second dose.
In both studies, the chance of infection after full vaccination was about 0.05 percent. The authors noted that the study findings are even more remarkable considering participants were likely to have a higher risk of exposure than average people because they work in health care settings. What's more, they were tested during a post-holiday surge of cases.
What can cause a breakthrough case?
Several factors can affect a vaccine's real-world effectiveness and the chance of a breakthrough case, experts say. Among them:
- Imperfect vaccine administration. It's rare, but if the vaccine is mishandled, it could cause a breakthrough case. Maybe the vials aren't kept at the required temperature, the vaccine is administered to the wrong part of your arm, or you don't get a full dose. In February, for example, a CVS pharmacy in Massachusetts issued an apology because it inadvertently gave some patients only a partial dose.
- An individual with a weak immune response. With every vaccine, there is a small subset of people who don't develop a robust immune response, says Chris Woods, an infectious disease physician and executive director of the Hubert-Yeargan Center for Global Health at Duke University. It could be the result of medications that weaken the immune system (such as chemotherapy for cancer) or genetic differences. Although age is also thought to weaken the immune response, the clinical trials for the COVID-19 vaccines show high efficacy rates in older adults.
- New coronavirus strains or variants. Early data indicate the current COVID-19 vaccines should work against known coronavirus variants that are more infectious. But it's possible that a variant could elude some of the protection we get from vaccination, Poland says. Fauci has said it's important to sequence the genome of the virus in the breakthrough cases to find out whether each infection is from the original virus strain or a variant. Meanwhile, vaccine manufacturers are already working on changes to provide better protection against variants.
The CDC is continuing to work with state and local health departments to track breakthrough cases that result in severe illness. The agency says it’s also working to identify any patterns in patient characteristics or specific variants that could have caused the infection.
Michelle Crouch is a contributing writer who has covered health and personal finance for some of the nation's top consumer publications. Her work has appeared in Reader's Digest, Real Simple, Prevention and The Washington Post.